Powder metallurgy (P/M) is a well-established process for the fabrication of near-net- shape components. In press and sinter applications for example, the powder is compacted in a die to form a green compact. The compact is then ejected from the die and sintered to create metallurgical bonds between the particles. For the compaction of the powder, a lubricant is generally required to improve the compressibility of metal powders and also to reduce the powder/die wall friction in order to facilitate part ejection and minimize die wear.
Processing of powders strongly depends on powder flowability. Powder flowability is defined as the time required for a specific quantity of powder to flow through an orifice or a die cavity. Flowability of a powder is important in high-volume manufacturing, which depends on rapid, uniform, consistent filling of die cavity. Poor flow characteristics cause slow and nonuniform press feeding and difficulty in ensuring a fill of the die cavity. Free-flowing powder refers to powders that readily flow in the die cavity [ASM Handbook, vol. 7: Powder Metallurgy].
It is well known that aluminum powders do not have good flowability, partly because of their low density. This is particularly critical when small particles, especially particles smaller than 50 .mu.m, are used. Flowability of the powder may be improved by using spherical powders. However, the flowability of spherical powder is in some case not sufficient for effective processing of the powder.
Particle agglomeration, also known as granulation and particle size enlargement, has been used for a long time to improve the flowability of powders. It is used for example in the fertilizer, pharmaceutical, food and mining industries. Different methods exist to form particle agglomerates. Some of them use a binder to agglomerate the particles.
The properties of aluminum P/M materials are highly sensitive to the presence of non- metallic additives in the starting powder. For that reason, the choice of an adequate binder that does not affect the properties of the final product is critical for aluminum P/M applications. The binder should burn out cleanly at temperature typically lower than 450.degree. C. to avoid the reaction of the decomposition products or residual products with the aluminum matrix. In addition, the binder content should be kept as low as possible to minimize the deleterious effect of the binder on the final properties of the material. On the other hand, the binder content must be sufficient to allow the formation of particle agglomerates. If the binder content is not sufficient, it is difficult to form agglomerates and the mechanical properties of the agglomerates are not sufficient for handling and shipping.
For the compaction of powder by P/M techniques, a lubricant is also generally required to ease the compaction of the powder and the ejection of the fabricated parts. The requirements for the lubricant are similar to those for the binder. The lubricant should not affect the final properties of the material.
Synthetic amide waxes, like ethylene-bis-stearamide wax, are frequently used as lubricants for aluminum press and sinter applications. A recent work of the instant inventors revealed that polyethylene wax may also be used for the compaction of aluminum powder compacts. As it is the case for the binder, the amount of lubricant should be minimized to optimize the properties of the final component. On the other hand, the lubricant should be sufficient to provide adequate lubrication during compaction and ejection.